The overall objectives of this project are to study the phenomenology and biological substrates of contingent drug effects. Using a kindling paradigm, we have demonstrated that the anticonvulsant efficacy of carbamazepine, and other drugs, could be manipulated by temporal factors relating to drug administration and seizure presentation. Significant findings to date include demonstration of the following: 1) contingent inefficacy, whereby the contingent presentation (i.e., before, but not after electrical stimulation) of carbamazepine during amygdala kindling seizure development, while not affecting kindling development, produced a subsequent refractoriness to carbamazepine's anticonvulsant effects on completed kindled seizures (when it should have been effective); 2) contingent tolerance, in which animals that have completed kindled seizures develop tolerance to carbamazepine following repeated administration of the drug prior to, but not after, each electrical stimulation; 3) contingent tolerance reversal by treatment with carbamazepine after the kindled seizures or kindled seizures alone (no drug), but not by time off (no drug or kindling stimulation) for periods of up to three weeks; 4) contingent refractoriness to valproate, in which animals that were kindled with the contingent presentation of valproate before each stimulation (which slowed kindling development), became valproate non-responsive; those kindled with non-contingent exposure to valproate remained sensitive to its anticonvulsant effects; 5) cross tolerance to carbamazepine in valproate-refractory rats; and reversibility of this effect by kindling the animals with valproate after each stimulation for one week; 6) cross tolerance between carbamazepine and a ligand that binds the peripheral-type benzodiazepine receptor and valproic acid, but not between carbamazepine and diazepam, clonazepam or phenytoin; 7) alterations in seizure threshold which mirror the changes in responsivity to carbamazepine; 8) slowing of contingent tolerance development by non-contingent drug presentation or by kindling the rats at lower stimulation currents, but not by higher doses of carbamazepine; 9) modulation of kindled seizure thresholds by different levels of kindling stimulation; 10) no effect of the NMDA antagonist MK-801 or the calcium channel antagonist nimodipine on contingent tolerance development.